1. Field of the Invention
The present invention pertains to an apparatus for detecting the passage of objects composed, at least in part, of ferrous materials such as iron or steel in crop harvesting machines.
2. Description of the Prior Art
Generally, prior art devices for detecting the presence of metallic, ferrous, or other objects have one or more inherent disadvantages rendering them of little value in certain applications, especially in the reliable protection of expensive crop harvesting machinery from tramp metal. Such prior art devices often operate with high excitation frequencies and are quite susceptible to the presence of moisture, various types of vegetation, vibration, or the like. They, therefore, impose on the operator the serious burden of continuous monitoring and adjustment of the calibration of the device. Even at relative low excitation frequencies such detectors are adversely affected to a significant extent by an over-extensive scanning area, thereby rendering them unusable for use where high accuracy, reliability and freedom from generation of false alarms is demanded. Additionally, prior art detection devices are located at positions which, for one reason or another, may fail to adequately detect the presence of incoming foreign objects. An example of a detection device located at a position within a crop harvesting machine, which has been found less than satisfactory, is illustrated in the U.S. patent issued to Bennet, Jr., et al., U.S. Pat. No. 3,757,501. This patent teaches the use of a detection device mounted in the floor plate of the header used with the harvester. This type of positioning permits undesirable objects which may be located on top of the incoming mat of crop material to go undetected. Further, this type of positioning creates the situation where each attachment, i.e. the headers, would have to incorporate the detecting device, rather than it being standard equipment in the body of the forage harvester, thereby increasing the cost to the farmer.